Process for removing an oxide layer from the surface of hardened strip steel by an electrolytical method



June 17, 1969 c E H. FRODEN 3,450,610

PROCESS FOR REMOVIN G AN OXIDE LAYER FROM THE SURFACE 0F HARDENED STRIPSTEEL BY AN ELECTROLYTIGAL METHOD Filed Nov. 30, 1964 Sheet Of 3 I' a),I\

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m i N ll I I ='\m A m N IN. P 0- INVENTOR, Carl Er/k Ho/ger Froden P 412PW W June 17, 1969 Filed Nov. 30, 196;

E. H. FRODEZN 3, 50,610

PROCESS FOR REMOVIN G AN OXIDE LAYER FROM THE SURFACE OF HARDENED STRIPSTEEL BY AN ELECTROLYTICQ}? METHOD 0 r 1 I 0 1 2 3 4 5 5 VO/f INVENTOR.Carl Erik Holger Frbden P 142% M? MW W June 17, 1969 c. E. H. FRCDEN3,450,610

. PROCESS FOR REMOVING AN OXIDE LAYER FROM THE SURFACE OF HARDENED STRIPSTEEL BY AN ELECTROLYTICAL METHOD Filed Nov. 30, 1964 Sheet ,3 of s sek.

1 T0 Ccul Erik Holggf' Frfiden PW WM & PM,

United States Patent US. Cl. 204145 3 Claims ABSTRACT OF THE DISCLOSURESurface oxide electrolytically removed from hardened and tempered stripsteel whilst latter moves as anode through electrolyte past a cathode.Arfter oxide is removed, resulting oxide-free surface is passivated.Electrolyte contains in excess of 20% by weight of chromium oxide and inexcess of 60% by weight of phosphoric acid.

This invention relates to a process for removing by an electrolyticalmethod an oxide layer [from the surface of hardened and tempered stripsteel that is continually conveyed through an electrolyte, with thestrip steel acting as the anode, and with one or several cathodesdisposed in the electrolyte in such a way that the oxide layer isremoved when the strip steel passes the cathode or cathodes.

In the process of the hardening and tempering of strip steel an oxidelayer is normally formed upon the surface of the strip, which results ina stain on the strip. In such cases as a bright surface is desired thisoxide layer has to be removed. Formerly, such a removal of the oxidelayer was performed mechanically, e.g. by means of a brushing operation.When dealing with thin strips, about 0.30 mm. or less, the mechanicalremoval of the oxide layer may cause certain disadvantages. Thus, thestrip may turn out uneven owing to the heat of friction, and in the caseof very thin material (about 0.07 mm. or less) further difficulties mayarise in the shape of fractures, fissures, or the like of the stripsteel.

By the mechanical removal of the oxide, a surface with a specificappearance is achieved, e.g. a brushed surface, which is not alwaysdesirable. If, for instance, the coldrolling which has preceded thehardening and tempering process has been performed in such a way that avery bright and scratch-(free surface or a specific dullrolled surfacehas been obtained, this surface will be altered in an undesirable lwayowing to the mechanical removal of the oxide layer.

At times it may be difficult to remove the oxide layer by mechanicalmeans, and on account of this the mechanical removal of the oxide layermay take a considerable length of time and may be expensive.

In the chemical or electrolytical process of the pickling, polishing orfinishing of steel and metals the oxide layer is removed at the sametime as a certain corrosion of the underlying metal takes place. Thesaid corrosion causes great disadvantages especially in respect of thinstrips when even an insignificant pickling effect may imply a seriousdrawback. Furthermore, in the process of the pickling and polishing ofhardened steel it has not so far been possible to avoid subjecting thematerial to considerable embrittlement owing to the absorption ofhydrogen.

From e.g. the German patent specification No. 763,900 it is known that apickling and/or polishing effect appears after a long time (10-30minutes) if low concentrations Olf Cr0 mixed with phosphoric acid areused. The German 3,450,610 Patented June 17, 1969 patent specificationrelates solely to batch treatment and not to a continual process.

The effect of the chromic acid in this connection is not commented uponin the abovementioned specification. A British patent specification No.526,854, which also deals with electrolytic polishing but according towhich an electrolyte consisting of phosphoric acid, sulphuric acid andchromic acid is used, indicates that the chromic acid has been added inorder to increase the brightness of the steel surface.

The process according to the present invention serves the purpose ofeliminating the inconveniences encountered in previously employedmethods, and it is mainly characterized in that, after the oxide layeralone has been removed, when the strip steel passes the cathode orcathodes the surface of the underlying strip steel is passivated bymeans of a CrO -content of at least 10% by weight in an electrolytewhich, in addition to CrO and Wager, consists of at least 50% by weightof phosphoric aci Although the theory of passivation has not beenexplained in every detail it is at present considered that the mechanismconsists in the creation of a multi-atomic but very thin, (about 50 A)dense oxide or hydroxide layer which prevents diffusion.

It is already known that such layers of passivation in non-alloyedC-steel can be produced by strong oxidizing agents alone, e.g. nitricacid or chromic acid, normally in high concentrations.

The passivation depends upon several factors. Thus, it was previouslyconsidered that normally no passivation of steel is met with in strongacids.

An elevated temperature and an increased current density in theelectrolytical process also render the passivation of the anode moredifficult in the presence of strong acids.

Phosphoric acid was proved suitable for the removal of the oxide layer.In order to increase the speed of the process, the strip steel isconnected as the anode in a circuit. When only phosphoric acid is used,however, the steel lying under the oxide is also corroded with or without the useof current.

It has now unexpectedly been found that an addition of chromic acid ofat least 10% by weight to a solution which, after this addition,contains at least 50% H PO (sp. gr. 1.75) in water, effectively retardsthe corrosion upon the surface of the steel from which the oxide hasbeen removed, both with and without the use of current. The followingcomposition of the bath is suitable, the percentages being by weight:

H PO 5068%, preferably above 60% CrO 1028%, preferably above 20% HO410%, preferably below 20% The most suitable composition has proved tobe:

Percent H PO 65 CrO 25 H O-4-40%, preferably below 20% Percent Carbon0.60-1.35 Silicon 0.10-2.00 Manganese 0.20-1.50 Chromium 0.00-0.40Nickel 0.00-2.25

and iron and impurities in amounts which are normal for such steel.

It has been found that contents higher than those indi cated above asthe maximum limits, render it difficult or impossible to restore theinitial surface. The lower limits are fixed with due regard to thenormal minimum limits for the respective material contained in hardenedC-steel. A C-content below 0.60% may easily be considered per se butsince the method relates to the removal of an oxide layer from hardenedstrip steel, a lower limit is of little interest. Example.

An example of the application of the present invention is shown in thefollowing:

The material which consists of hardened and tempered strip steel havingthe dimensions 12.7 x 0.04 and a composition of:

percent by weight C 1.20 Si 0.23 Mn 0.35 Cr 0.13

the remainder consisting of iron and the usual impurities and which iscovered by a thin oxide layer, is continually passed through anelectrolyte of the composition shown below, and in which the steel isconnected as the anode.

The equipment for the electrolytical removal of the oxide is shown inthe attached drawing.

and

After such a treatment a surface completely free from oxide is obtainedwithout the slightest visible corrosion upon the surface of the steelitself.

The current density, composition of the bath, temperature of the bath,length of time in the bath, may be varied within certain limits, andthey may be adapted in such a way that only the oxide layer is removedand the underlying surface is not corroded at all or only very slightly.Thus, on the whole by this treatment the surface finish and the surfaceappearance of the unhardened steel will be restored. Consequently, thereis no electrolytical polishing of the strip steel material.

The relation between current density and voltage is shown in theattached FIGURE 3. For cell voltages between -2.6 volts the currentdensity increases considerably in the area a-b. Between b and c there isan area where both voltage and current density are very unstable.Between c and d the current density remains constant, while it increasesvery much between d and e.

If voltages to the left of a are applied the oxide layer upon the anodeis not dissolved. Between a and b the oxide begins dissolving but inthis area an etching effect is obtained. Also between b and c, a certainetching of the surface of the anode is obtained. Between 0 and d, asmooth dissolution of the oxide is obtained without any trace of etchingeffect on the surface of the anode. Thus, point e on the curverepresents the lowest voltage (2.9 volts) that can be applied. On thed-e sector of the curve the same results are obtained as on the c-dsector but, since here the current density greatly increases for thehigher cell voltage, the dissolution of the oxide is more rapid. Thecurrent density limits cannot be determined without taking the length oftime into account. It has been found that, for the d-e sector of thecurve, which is the sector of the current density/voltage curve used inpractice, the relation between current density and length of time is theone which is shown in FIG. 4 at a temperature of 25 C. Times below A-Bdo not result in a complete dissolution of the oxide, while times aboveAC produce an incipent etching effect. With the present power supplymethods current densities above A/dm. seem to be very diflicult to usein a continuous line owing to the risk of flash-over between the stripsteel and the power supply rolls.

The values indicated in FIGURES 3 and 4 are not generally applicable toevery type of bath composition and to all temperatures; they are givenmerely as examples of the possible variations of thes factors.

An appliance for the carrying out of the process of the presentinvention may be arranged in such a way that strip steel is conveyedthrough one or several vessels containing an electrolyte, the stripsteel acting as the anode. One or several cathodes are arranged in theelectrolyte.

The present invention is described in the accompanying drawing whichillustrates a practical embodiment of the electrolytical treatmentdescribed above.

FIG. 1 shows in a schematized manner a section of an appliance seen fromthe side, and

FIG. 2 shows the same appliance seen from above.

FIGS. 3 and 4 have been referred to above.

Strip steel 1 comes from a coiling not shown here and is conveyed in thedirection of arrow 4 through an existing electrolyte 3 in a vessel 2 ofsuitable type. On its passage through the electrolyte the strip steelpasses a number of conductive rolls, so-called current rolls. In theexample shown these are five in number and they are designated 5, 6, 7,8 and 9. From the last current roll 9 the strip steel is conveyed to arinsing and drying appliance, not shown in the example. The currentrolls are arranged above the upper surface of the electrolyte. In orderto lead the strip steel into the electrolyte breaking rolls 10 arearranged in the bath. Two such breaking rolls are placed between eachadjacent pair of current rolls. Between each pair of the breaking rollstwo starting sheets 11 and 18 are arranged, one on each side of thestrip steel.

The current is fed via bars 19 and 20 directly to the current roll 9 andthe cathodes 17 and 18. The other cathodes 11-16 are connected to thecathodes 17 and 18 by means of busbars 21. Busbar 19 is connected to thedistribution bar 22. The current rolls 5-8 are connected to thedistribution bar 22 via the series resistances 23-26.

The current rolls are preferably made of stainless steel but they couldof course be made of any other suitable material. The current isconducted to the strip steel 1 through the current rolls, and thematerial in these shall consequently be conductive.

The cathodes 11-18 consist of stainless steel plates having a greaterwidth than that of the strip steel and they are placed on each side ofstrip steel 1 at a distance of 10-30 mm., preferably 20 mm. from eachother. The cathodes could of course also be made of any other resistantconductive material.

The current density is mainly controlled by means of the appliedvoltage.

The speed of the strip steel depends upon the current density and thelength of the appliance. With an effective length of the bath of about2.5 metres the speed can suitably be between 3 and 6 metres per minute.The speed depends of course upon the thickness of the oxide layer.

According to the process of the present invention it is possible e.g.continually to remove the oxide layer from hardened and tempered thinfeeler gauge teel having a thickness of 0.02 mm. while maintaining theextremely narrow tolerances required for this type of steel.Consequently it is already possible to give the material its correct andfinal thickness during the cold-rolling process.

What I claim is:

1. A process for removing an oxide layer by an electrolytical methodfrom the surface of hardened and tempered strip steel that iscontinually conveyed through an electrolyte, the strip steel acting asthe anode, and with one or several cathodes disposed in the electrolytein such a way that the oxide layer is removed when the strip steelpasses the cathode or cathodes, in which process, after the oxide layeralone has been removed, the surface of the underlying strip steel inpassing the cathode or cathodes is passivated by aprO -content of 20 to28% by weight in an electrolyte which, in addition to CRO and water,consists of more than 60%, but not more than 68% by weight of phosphoricacid.

2. A process according to claim 1 wherein the strip steel has thefollowing composition:

and impurities in amounts which are normal for such steel,

the balance being iron.

3. A process according to claim 1, wherein the temperature of theelectrolyte is below centigrades.

References Cited UNITED STATES PATENTS 2,334,699 11/1943 Faust 2042,347,040 4/ 1944 Faust 204-1405 2,366,712 1/1945 Faust 204-l40.53,066,084 11/1962 Osterman et al 204-144 JOHN MACK, Primary Examiner. W.B. VANSISE, Assistant Examiner.

US. Cl. X.R. 204-34, 140.5

